Method and device for controlling a short circuiting type welding system

ABSTRACT

A method and device for controlling a power supply for arc welding in a manner to reduce spatter when the power supply is employed for depositing metal from a welding wire or electrode onto a workpiece by the short circuiting transfer mode wherein a welding current causes the welding wire to alternate between a short circuit condition and an arc condition with metal transfer occurring during a short circuit condition. This method and device includes the concept of shifting the welding current to a background current value in response to a short circuit condition, holding the welding current generally at the background current level for a preselected time, then allowing the welding current to reach the normal unimpeded current level, and causing the holding step to be terminated before the selected time in response to a detected arc condition. This concept provides a predetermined low current condition immediately upon establishing a short circuit between the welding wire or electrode and the workpiece, which low current condition is retained long enough to convert what otherwise would be a spatter-laden momentary short circuit to a short circuit where metal is transferred to the workpiece. Further, the method and device detects the slope of the welding current or voltage and shifts the welding current to the low background current level when the welding current reaches its maximum value just before breakage of the metal from the wire thus reducing the spatter energy when the molten metal breaks from the wire.

This is a division, of application Ser. No. 940,580 filed Dec. 11, 1986U.S. Pat. No. 4,717,807.

DISCLOSURE

The invention relates to the art of welding with an electric arc andmore particularly to an improved method and device for controlling ashort circuiting type welding system to drastically reduce spatter whichnormally accompanies this type of welding process.

BACKGROUND OF INVENTION

In consumable electrode arc welding, one of the recognized modes ofoperation is the short circuiting mode, wherein a power supply isconnected across the consumable electrode, or welding wire, and theworkpiece onto which a weld bead is to be deposited. As an arc iscreated, the end of the electrode melts to form a globular mass ofmolten metal hanging on the electrode and extending toward theworkpiece. When this mass of molten material becomes large enough, itbridges the gap between the electrode and the workpiece to cause a shortcircuit. At that time, the voltage between the electrode and theworkpiece drops drastically thereby causing the power supply todrastically increase the current through the short circuit. Such highcurrent flow is sustained and is actually increased with time throughthe molten mass as the power supply inductance is overcome. Since thisshort circuit current continues to flow, an electric pinch necks down aportion of the molten mass adjacent the end of the welding wire. Theforce causing the molten welding wire to neck down is proportional tothe square of the current flowing through the molten metal at the end ofthe welding wire. This electric pinch effect is explained by theNorthrup equation: ##EQU1## I is current density, r is the distance fromthe center of the welding wire and R is the diameter of the neck. Duringthe short circuit, there is a need for a relatively high current flow,which flow naturally results when the short circuit occurs. This highcurrent flow is desirable to cause the neck portion of the molten massto form rapidly into a very small area or neck which ultimately explodeslike an electric fuse to separate the molten ball from the wire andallow it to be drawn into the weld pool by surface tension. Thisexplosion of the neck causes spatter from the welding process. Spatteris deleterious to the overall efficiency of the welding operation andrequires a substantial amount of cleaning adjacent the weld bead afterthe welding operation is concluded. Since the current flow through thewire or rod to the workpiece when the neck or fuse explodes is quitehigh, there is a tremendous amount of energy released by the neckexplosion adding to the propelled distance and amount of spatter.

As can be seen, there is contradiction between the short circuit currentwhich should be high to efficiently decrease the neck size by anelectric pinch, but should be low to reduce the energy of the fuseexplosion and, correspondingly, reduce the spatter and distance overwhich the spatter particles will be propelled.

A considerable amount of effort has been devoted to limiting spatterwhen the arc is reestablished by the explosion at the neck or fuse ofthe metal ball hanging from the welding wire and engaging the workpieceor weld pool. At first, it was suggested to reduce the diameter of thewelding wire, i.e. use a 1/32 wire; however, this approach to reducingspatter caused all of the inefficiencies normally associated with usingsmall welding wire. For instance, it was difficult to lay large amountsof weld bead and the wire sometimes stubbed or entered the weld poolwithout melting. As the wire diameter increased to overcome theseproblems, spatter was substantially increased. Faced with this dilemma,it was suggested that a high frequency power supply be used as taught inU.S. Pat. No. 4,544,826, incorporated by reference herein, wherein ahigh frequency inverter is turned off during a short circuitingcondition or upon detection of a premonition of rearcing, i.e. blowingof the fuse. To prevent circulating currents when a high frequency powersupply is turned off just before a fuse explosion, this Unites StatesLetters Patent illustrates a switch, SWD, which is opened to place aresistor in the output tank circuit of the solid state inverter forrapid attenuation of the circulating currents. This system is notapplicable for all power supplies and is predicated upon a complex logiccontrol system which actually forms the shape of the current curve fromthe time a short is detected to the time when the arc is reestablishedafter explosion of the neck or fuse. Reduction of current at the time ofa short is by tuned attenuation, which phenomenon causes a time constantcurve between time t₁ and t₂. At the detection of a neck or fuse whichis about to blow, this same attenuation concept is employed. Thisfeature is shown between the times t₅ and t₆ of this prior patent. Thepreselected wave shape, as shown in this patent, is heavily reliant uponthe aforementioned attenuation of the output tank circuit of a solidstate inverter which is a serious limitation especially in reducing thecurrent flow through the neck itself at the moment of explosion. Such apreselected current shaping is applicable, if at all, to a highfrequency solid state inverter power supply which can be internallyturned off without substantial output inductance. With a substantialinductive reactance in the output circuit attenuation by the resistor inparallel with switch SWD would be difficult and not always guaranteed.Since direct current welding systems have output inductance thisattenuation concept for lowering spatter has serious practicaldrawbacks.

Another patent showing a system for creating a repetition of a currentcycle originally triggered by a short circuit detection is U.S. Pat. No.4,546,234. Again, the current wave form is somewhat fixed. After apreselected time delay, current is applied across the shorted moltenmetal globular or ball to facilitate metal transfer. A constant currentis maintained until necking is predicted, at which time the currentdrops rapidly to a low level and then immediately shifts up to a secondhigh level. This system causes preselected current wave forms which arecomplex and generally usable, if at all, only with a high frequencysolid state inverter type power supply.

As can be seen, there is a definite need for a relatively simplifiedsystem for reducing weld spatter by exerting a limited amount of actualcontrol over weld current flow so that the current flow can assumenatural operating characteristics over most of the cycle between theshort and the fuse explosion. In addition, there is a substantial demandfor a spatter reducing circuitry to be used with both transformer fedand solid state inverter type power supplies which do not depend uponoutput attenuation of low inductance circuits not upon several distinctcurrent level limitations.

THE PRESENT INVENTION

The present invention overcomes the disadvantages of prior attempts toreduce spatter in a welding system of the type employing the shortcircuit, transfer mode which system requires a minimum of logic circuitsand is applicable for a wide variety of power supplies with and withouta substantial amount of inductance in the output circuit.

In accordance with the broadest aspect of the present invention, a mainweld current is on whenever the arc voltage exceeds a preselectedthreshold level, such as 10 volts. When thea arc voltage drops belowthis preselected value, the main welding current is turned off for aselected time and is then turned on again. During this period or cycle alow background current is maintained so the molten metal mass or ballhanging from the welding wire and in contact with the workpiece eitherbreaks away or develops into a metal transfer short under the influenceof only the low background current and not the main current.Consequently, any molten metal ball which does not actually transfer tothe weld bead or weld pool on the workpiece will be subject to only lowbackground current when it separates from the weld pool. Such lowcurrent does not tend to propel the ball or portions thereof from thewire away from the weld pool. These bulbs or balls of molten metal mayonly momentarily engage the molten weld pool or weld bead thus causing aphenomenon referred to as an "incipient short". An incipient short isnot a metal transfer short, but is the engagement of the ball with theweld pool succeeded by a bouncing away of the ball from the molten poolby electric pinch forces to again establish an arc without any metaltransfer. The momentary short would occur well within the selected timeof low current. In practice, this time is 1.0 ms. As incipient shortsare created, the main weld current is turned off reducing pinch forcesat the contact point between the molten metal ball and the weld pool. Bymaintaining the welding action with a low level current flow there issufficient current to maintain the melting action but generallyinsufficient current to create high level pinch forces tending tore-establish the arc allow arc jet forces to propel the molten ball fromthe weld pool. Consequently, the short converts into a metal transfershort and progresses without forming an incipient short. The term"workpiece" is used herein to indicate either the metal onto which theweld bead is being laid or deposited, the bead itself or the weld pool.All of the these are electrically grounded to the power supply.

If the short circuit is only a momentary short associated with anincipient short condition, the amin current will come on when the shortis broken and the arc reestablished. If the incipient short condition isconverted to a transfer short during the time of forced low current,which is the general result, a short condition remains after the initialpreselected time of low current. The arc voltage remains at a low level,but the main current is turned on. When this occurs, the current throughthe molten bath between the welding wire and the workpiece increasesrapidly because of the continuing short circuit between the welding wireand the workpiece. As main current continues to flow through the weldingwire, the wire continues to heat and its resistivity increases. The ballbridging the gap starts to neck down by the electric pinch effect at arate proportional to the square of the weld current. As the resistivityincreases and as the neck decreases in diameter, the voltage commencesto increase. Since the necking action is generally self-sustaining afterit starts, commencement of the neck signals an impending fuse blowpreceded by an increased voltage and a change in sign of the currentslope. A time derivative of the operating voltage or of the weld currentwith the main current applied across the shorted metal indicates whenthe metal is electrically pinched by the main current flow. A rapid risein the voltage or change of slope of the weld current indicates animminent blow of the fuse or neck. When an imminent fuse explosion isindicated by the voltage or current derivative, the main current isagain immediately turned off, just before the loss of metal contact.With the main current off, the low background current causes the fuseexplosion at the neck. This is a low energy explosion withoutsacrificing efficient separation. During separation of the bridgingmetal, the plasma or arc restored by the background current and the arcvoltage increases. When the arc voltage exceeds the control value, themain current is again turned on awaiting the next short.

To prevent interruption of the main current immediately after a short isdetected, the circuit measuring the time derivative of the voltage orcurrent deactivated for a short time immediately following the firsttime delay after a short circuit detection. This deactivation prevents aderivative measurement or detection when the main current is turned onafter the time delay. If this derivative detection feature were tofunction immediately after the delay, the main current would again beturned off thereby preventing the formation of high shorting current anddevelopment of a strong electric pinch. During this transition to themain current there are variations in the voltage which could beerroneously identified as a necking condition by the circuit measuringthe time derivative of the arc voltage or time derivative of the weldcurrent.

The invention as defined above allows the main current to switch off sothat a low level background current is maintained for a preselectedmaximum time immediately after a short circuit detection. Upon detectionof an imminent fuse breakage at the end of a metal transfer cycle thesame low level background current is applied for a selected time delay.During these time delays, should the voltage increase above the selectedvalue, thus indicating an arc condition, the main current is immediatelyapplied and the welding process continues as if there were no controlover the weld current.

Actual metal transfer in a short circuiting mode of operation follows ashort circuit condition caused by the molten ball touching the weldpool. After the short circuit, the flow of current drastically increasesthrough the shorted ball until the current flow causes a necking of themolten metal ball at the end of the weld wire. When that happens, theresistance through the molten metal ball increases causing acorresponding decrease in the applied main current. Immediatelythereafter, since the main current is still flowing, the neck sizedecreases until it explodes. By employing the present invention, thecurrent actually flowing through the neck or fuse when it explodes isreduced to a level drastically below the normal current levelexperienced by using the main welding current during actual fuseexplosion. A reduction in current flow at the time of rupturedrastically reduces spatter by decreasing the energy of the fuseexplosion. By using a derivative of voltage or current, the neck can beaccurately detected so the current can be reduced before the fuse blows.

In accordance with the present invention, the first time delay isinterrupted and the main current is immediately applied whenever an arcis reestablished after a short. This occurs during an incipient short.The main current then increases to the plasma level awaiting an actualtransfer short. Consequently, the metal ball at the end or the weldingwire is not subjected to high propelling forces. The molten balls do nottend to grow by repeated momentary shorts separated by arc jets. In thismanner, the present invention recognizes and overcomes the problemscaused by incipient shorts by allowing actual metal transfer to the weldpool during normal transfer shorts, but also providing only a lowcurrent condition during the formation of incipient shorts. Thus, highcurrent flow does not cause drastic high activity breaking the moltenball from the weld pool as occurring in a system with no reduction incurrent at the start of a short. However, the present inventionoverrides the low or background current control feature whenever thereis an arc, immediately following a short circuit condition, such aswould occur with an incipient short, if one should appear. By using theinvention, incipient shorts are generally avoided. Further, the weldcurrent is not forced through a preselected combination of currentlevels based purely on time cycles, as sometimes employed in the priorart for reason other than control of incipient shorting.

In summary, the concept of incipient shorting as a mechanical componentof spatter and utilizing the present invention to eliminate nearly allincipient shorts is a substantial improvement in spatter control. Inaccordance with the invention, every short is assumed to be an incipientshort and the welding current is reduced to a background level so theagitation of the weld pool and the forces of the arc are minimized. Thegreatly increases the probability that even an incipient contact betweenthe molten ball and the molten weld pool will be converted into anormal, desirable transfer short by attraction of the weld pool surface.Should the incipient contact or short be one which is abnormally violentand cannot, or does not, convert into a transfer short, the fact thatthe ball contacts and separates from the weld pool at the low backgroundcurrent level all but eliminates spatter normally generated by thisnontransfer action referred to herein as an incipient short.

By recognizing and correcting the disadvantages of the incipient shortsand by also correcting the problems of high energy fuse explosion withonly a shift between main current and background current, the problem ofweld spatter has been essentially eliminated.

By utilizing the concept of the present invention, spatter can bereduced drastically over normal short circuiting type welding withoutthe complexity and current wave shaping of other spatter reductiontechniques.

Use of the present invention reduces the violent oscillations of themolten weld pool which further reduces tendency of spatter. Further, amore quiescent weld pool allows the surface tension to better bridge theroot gap between two pieces being welded. Also, the pool will betterconform to the pieces being welded with the reduced agitation caused byhigh energy fuse explosions and incipient shorts. The weld pool, whenless agitated, allows use of larger electrodes in out of positionwelding. Short arc lengths can be maintained without stubbing.Consequently, the invention allows use of larger electrodes, highdeposition rates, high currents and, also, less contamination byentrapment of shielding gases.

The primary object of the present invention is the provision of a methodand device for controlling current between a main welding current and abackground welding current in a manner to drastically reduce spatter ina short circuiting type of welding system.

Another object of the present invention is the provision of a method anddevice as defined above, which method and device does not control thewelding current in a series of preselected levels having imprecisecorrelation with the actual demand for reduced spatter.

Another object of the present invention is the provision of a method anddevice, as defined above, which method and device prevents incipientshorts from causing large particle spraying from the welding operation.

Yet another object of the present invention is the provision of a methodand device, as defined above, which method and device allows use of alarge range of wire diameters. Consequently, larger welding wire can beused without deleterious spatter. The terms "wire" and "electrode" areused somewhat interchangeably to mean the elongated consumable metalelement feed into the welding area to be transferred during the weldingprocess. The invention may be used with manual stick electrodes.

Another object of the present invention is the provision of a method anddevice, as defined above, which method and device can be used on powersupplies of the inverter type, motor generator sets, and conventionaltransformer type. In the past, spatter control systems were primarilylimited to high frequency solid state inverters because these powersupplies exhibited substantially reduced inductive reactance in theoutput circuit. The spatter control systems of the past required lowoutput inductive reactance.

Still a further object of the present invention is the provision of amethod and device, as defined above, which method and device employs aDarlington power transistor. This type of transistor has a rapidturn-off time and a high voltage and current rating.

Still a further object of the present invention is the provision of amethod and device which allows high current during the transfer short toinitiate formation of a neck between the rod and the ball of materialbeing transferred and then, abrputly, reduces the current just beforethe neck acts as a fuse and explodes. This reduced current at the timeof "blow" or explosion produces a low energy during the actualrestriking of the arc by rupture of the neck. The high short circuitcurrent at the initial stage of forming the neck causes a substantialelectric pinch. This advantage is confirmed by the known electricalprinciple that the electric pinch is a factor of the square of thecurrent flowing through the molten metal ball between the welding wireand the workpiece. Should current be turned off or reduced before theneck is well formed and nearing explosion, the desired neckingphenomenon would be adversely affected. The neck is started with a highcurrent and then the squeezing action at the neck continues with the lowcurrent applied directly a mere instant before the fuse ruptures.

Yet another object of the present invention is the provision of a methodand device for reducing spatter in a short circuiting type of weldingoperation which can function with a variety of shielding gases and withdifferent electrode types and sizes.

Yet another object of the present invention is the provision of aspatter reduction system which reduces both neck exposing spatter at theend of a metal transfer pulse and also incipient short spatter at thestart of the transfer pulse.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawingsdescribed below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram a short circuiting type welding systememploying the preferred embodiment of the present invention;

FIGS. 2A, 2B and 2C are schematic views illustrating progression of themolten metal bath formed on the end of the welding wire during the shortcircuit condition and at the fuse break;

FIG. 3 is a graph of the welding current during a somewhat standardmetal transfer, as shown in FIGS. 2A, 2B and 2C;

FIG. 4 shows voltage and current graphs detailing operatingcharacteristics of the present invention;

FIG. 5 is a wiring diagram of an alternate system for predicing theoccurrence of the fuse action or neck explosion as illustrated in FIG.2C;

FIGS. 6A, 6B, 6C and 6D are schematic views illustrating and "incipientshort", as this term is employed in the present application and certainphysical and electrical characteristics of this surprising phenomenon;

FIG. 7 is a graph showing welding current exhibiting certain electricalcharacteristics associated with the incipient shorts phenomenon, shownschematically in FIGS. 6A-6D;

FIG. 8 is a current curve showing two unwanted incipient shorts followedby a desired metal transfer short with the time abscissa expanded andinterrupted and the current ordinate somewhat exagerated;

FIG. 9 is a schematic diagram of the preferred embodiment of the presentinvention used in explaining a characteristic of the present inventionillustrated in FIG. 10;

FIG. 10 is a graph showing a welding current wave form using one featureof the invention for reducing current at the neck;

FIG. 11 is a wave shape graph showing a metal transfer pulse employingall aspects of the present invention;

FIG. 12 illustrates the preferred embodiment of the present inventionwith components for protecting the power Darlington transistor from highvoltage and high circulating current;

FIG. 13 is a wiring diagram of the actual circuit employed in practicingthe preferred embodiment of the present invention which diagram isdivided into two sheets labeled FIGS. 13A and 13B; and,

FIG. 14 is a schematic diagram of the welding current, arc voltage foreach state of the welding operation and contrasting a standard shortcircuit welding system with a system using the new spatter controlsystem.

PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing wherein the showings are for the purpose ofillustrating the preferred embodiment and not for limiting same, FIG. 1illustrates a welding system A constructed in accordance with thepresent invention. System A contains a spatter reduction or controlcircuit SC which will be desired later. Since spatter control circuit SCis capable of addition to a standard short circuit type welding unit,system A of FIG. 1 illustrates components common to a welding operationwhether or not inventive circuit SC is used. FIG. 1 will initially beemployed to explain general background and concepts used in shortcircuit type welding. The commonly used components of system A include aconventional transformer type power supply 10 having a constant voltageoutput with inductive reactance internal to the power supply thatprevents rapid discontinuation of welding current using normal solidstate switching procedures. Output leads 12, 14 are connected in seriesacross gas nozzle 20, welding wire 22 and workpiece 30. These componentsare arranged in a series circuit schematically illustrated in FIG. 1;however, a mechanism for feeding the wire 22 toward the weld pool onworkpiece 10 for the purpose of laying a bead along the workpiece wouldbe an integral component of system A. Such normal wire feeding mechanismdoes not form a part of the present invention. The present invention canbe used in a standard short circuiting mode of welding as so farexplained with the aid of standard components illustrated in system A ofFIG. 1. This system employs a conventional transformer type power supplyapplying and arc voltage across wire 22 and arc or plasma P. Suchvoltage causes a weld current to flow from wire 22 to workpiece 30. Inaccordance with general practice an appropriate shielding gas 40 isdirected from nozzle 20 around wire 22 to reduce oxidation andcontamination of the metal being deposited in the weld pool on theworkpiece 30. Of course, either workpiece 30 or welding head includingnozzle 20 is moved along a desired path to deposit a linear welding beadeither on the surface of the workpiece or in a groove formed between twoabutting workpieces.

In a standard short circuiting transfer mode of welding as so fardiscussed, the metal is deposited from the wire onto the workpiece bypassage of a welding current through the wire and to the workpiece whichalternates between a short circuit condition wherein the rod or wiretouches the workpiece and an arc condition or plasma condition, whereinthere is a gap between the wire and workpiece. This gap below the wireis spanned by an arc or plasma. The term "workpiece" refers to the weldpool already deposited on the surface being treated or the workpieceitself, which definition is used herein for convenience. FIGS. 2A, 2Band 2C, schematically illustrate the short circuiting transferphenomenon for transferring metal melted at the end of wire 22 anddesposited on the workpiece by the surface tension drawing the metalinto the weld pool of molten metal containing the previously depositedmetal. This weld pool is dynamic in nature and retains its fluidity fora substantial time after nozzle 20 progressively moves along theworkpiece. As is well known, the bottom of wire 22 is maintained spacedfrom the workpiece or molten weld pool previously deposited a distance bwhich is substantially greater than the diameter a of the wire. Inpractice, diameter a can vary substantially and still be used in awelding system using the present invention. In the past, small wirediameters, typically, 0.035 inches were used to reduce spatter sincesuch small wire was cradled in the weld pool and spatter, if any, wasphysically caught in the weld pool formed around the end of the wire asit deposits metal into the weld pool. Since spacing b is greater thandiameter a, a molten metal bulb or ball B, shown in FIG. 2A, has ageometric spherical shape somewhat greater than a hemisphere. This ballshape facilitates separation and surface tension transfer from rod 22 tothe molten weld pool or work piece 30. In summary, in the standard shortcircuit mode of welding, current passes through wire 22, and an arc iscaused between the bottom of the wire and the weld pool or workpiece 30,whereby the heat of the arc or plasma causes the end of wire 22 tobecome molten. This molten material grows in size until it bridges thegap indicated as spacing b by forming ball B and causes a short circuit.Since the circuit resistance drops drastically when this short circuitoccurs the weld current immediately increases and the voltageimmediately drops. The rapid current rise is shown at the left side ofthe metal transfer pulse MT, illustrated in FIG. 3 wherein the weldingcurrent over a normal short circuit metal transfer is schematicallyillustrated. Welding current I_(W) has a normal arc or plasma currentlevel I_(P) which is a minimum current level and is controlled primarilyby the output resistance. Between arc or plasma periods when plasmacurrent I_(P) is flowing, there is a metal transfer short pulse MT wherewelding current I_(W) is increased and then decreased back to plasmacurrent I_(P). Pulse MT, which is standard when a transfer short occurs,includes a leading, rapidly rising curved side 50, which increases to amaximum from the point S. A short condition is illustrated in FIG. 2A.As the current increases, ball B clings by surface tension to the moltenweld pool and froms a distinct neck N in accordance with the electricpinch effect. The diameter of the neck N is reduced by current inducedforces in accordance with the Northrup equation. This causes theresistance through ball B to increase. Consequently, when the neckstarts, the welding current I_(W) curve in FIG. 3 reverses direction andstarts downward due to the increasing resistance as the diameter of neckN decreases rapidly. Since the necking force or electric pinch effectvaries as the square of the welding current, substantial forces arebeing exerted at the top of ball B to cause a rapid necking down. Forthat reason, as soon as the neck starts as shown generally in FIG. 2B,the neck diameter decreases rapidly at an accelerated rate. This resultsin a fuse action or neck explosion F, represented schematically in FIG.2C. This explosion at point 54 in FIG. 3 immediately restrikes a plasmaor arc between wire 22 and workpiece 30 so that the current I_(W) firstdrops rapidly along portion 55 of pulse MT and then gradually along line56 as the arc stabilizes. At top 52 of metal transfer short pulse MT thecurrent derivative di/dt is reversed and becomes negative. As soon asthe neck is formed, it immediately breaks as illustrated by steep line57 between top 52 and explosion point 54. Time constant portion 56 ofmetal transfer pulse MT follows the initial rapid current drop alongline 55 immediately after the explosion at point 54. As can be seen, theneck starts forming and is immediately blown away in short time TN inFIG. 3, which is drawn to scale.

In summary, ball B forms on the end of wire 22 and enlarges until itmakes contact with the weld pool or workpiece 30 shown in FIG. 2A.Surface tension then draws ball B from the end of rod 22 preparatory tothe electric pinch effect causing ball B to neck down as shown at N inFIG. 2B. Immediately thereafter, the fuse breaks or blows, as shown inFIG. 2C. As shown in FIG. 3, at the fuse F, the current is nearly 300amperes; therefore, tremendous energy is released when ball B separatesfrom wire 22. This causes spatter of molten metal, indicated as arrowsSP in FIG. 2C. This spatter flies outwardly with a high momentumcarrying spattered metal some distance away from the actual weldingoperation. To reduce spatter SP in the past, wire 22 was reduced in sizeso as to bury the arc in the weld pool, thus, permitting the weld poolto catch most of the spatter particles of molten metal and reduced thetendency to drive the spatter particles away from the weld pool. Also,complex circuits were suggested to control the shape of metal transferpulse MT during the metal transfer cycle. These prior arrangementstended to drive pulse MT in a preselected pulse shape. Often the pulseswere repeated, irrespective of whether they were needed or not orwhether the metal transfer actually occurred. The present inventionovercomes these disadvantages by making only minor modifications in theconventional system so far discussed with respect to certain componentsof system A in FIG. 1. The normal welding current curve, as shown inFIG. 3, is controlled by simple, easily accomplished structuralmodifications of the normal short circuit system so far described.

Referring again to FIG. 1 and to FIG. 4, the components added incombination with the standard features of system A to accomplish thepreferred embodiment of the invention are illustrated in FIG. 1 wherespatter reduction circuit SC includes resistor R connected in parallelwith a Darlington power transistor switch SW. This parallel circuit isconnected in series with nozzle 20, wire 22 and workpiece 30. Tocomplete the preferred embodiment of the present invention, spatterreduction circuit SC also includes solid state logic control system Lfor operating switch SW. The manner of operating the switch inrelationship to parameters at the welding site substantially eliminatesspatter SP experienced in standard short circuit welding. Control Lcommands Darlington switch SW to be either conductive or non-conductive.When non-conductive, resistor R is in series with the welding operationto create a low level welding current hereinafter referred to as thebackground current I_(B). In practice, resistor R is 1.0 Ohms whichdevelopes approximately 300 volts across the resistor when switch SW isturned off while the main current flow is near 300 amperes.Consequently, power supply 10 is never actually disconnected or groundedto reduce the current flow through the welding operation irrespective ofthe status of switch SW. Merely switching between a direct connectionthrough switch SW and current flow through only resistor R causeswelding current I_(W) to either (a) float, in accordance with standardpulse MT or (b) be driven downward by opening switch SW with a logiccontrol L. One aspect of the present invention is the use of aDarlington connected power transistor having a rating of several hundredamperes and several hundred volts. The Darlington connected transistornetwork SN is preferred because of capabilities of rapid switching underheavy current conditions. This type switch is part of the presentinvention which must drop the welding current from a high level maincurrent to a low level background current I_(B) within less than about120 microseconds when the switch is opened or in the non-conductivecondition.

An appropriate voltage sensor, schematically represented as device 60,and a current sensor 62 provide logic control L with instantaneouswelding current I_(W) and arc voltage E_(ARC) so that logic control Lopens and closes switch SW to provide the weld current control asillustrated in FIG. 4. The basic aspects of the present invention areset forth in graphic form in the upper graph of FIG. 4, which graphillustrates voltage fluctuations. The lower graph illustrates weldingcurrent I_(W) through the wire and workpiece as correlated on a timebase with the upper voltage curve.

FUSE SPATTER REDUCTION

To appreciate one aspect of the present invention to reduce the fuseenergy to reduce spatter SP, attention is directed to the right hand endof the metal transfer pulse MT (NEW) in FIG. 4. This pulse has a frontcurved shape, side or line 100 correlating directly with portion 102 ofthe arc voltage which will be described in connection with anotheraspect of the invention used at the left end of pulse MT. As currentI_(W) increases to the right, the slope decreases eventually, whichchange in current at the right end of line 100 causes a slight increasein the voltage slope at portion 104. This increase in voltagecorresponds with the upper, generally flat portion of transfer pulse MT(NEW). As the current approaches a zero slope and when the voltageexperiences an increased slope, the neck N is starting to form, as shownin FIG. 2B. At this instance, in accordance with this aspect of thepresent invention, either by detecting di/dt or dv/dt, switch SW isopened. This immediately shifts the current to the low backgroundcurrent level I_(B) schematically illustrated as about 50 amperes inFIG. 4. The welding current stays at this low background level only solong as the arc voltage E_(ARC) does not exceed a preselected value,such as 10 volts illustrated in the upper voltage curve. Then the neckbreaks, the short circuit is removed and the voltage rises alongvertical line 106. When arc voltage E_(ARC) exceeds the selected lowlevel (i.e. in practice 10 volts) as it moves along line 106, switch SWis closed and the welding current I_(W) moves along a curve 110, whichcurve is a time constant curve raising to the plasma or arc weldingcurrent value I_(P), as shown at portion 112. This plasma current isillustrated as being substantially over 200 amperes. At the same time,the voltage assumes its steady state condition at portion 108,illustrated as somewhat over 20 volts.

Pulse MT (NEW) causes metal transfer from the welding rod to theworkpiece, i.e. the molten weld pool, as did the conventional metalpulse MT. The time T_(P) in FIG. 4 is the metal transfer portion of thepulse. The first aspect of the present invention, as explained above,occurs at the end of pulse MT (NEW) and involves recognizing when theneck starts to anticipate a fuse blow. Then welding current I_(W) isshifted to a low level substantially below the plasma level I_(P) byopening switch SW and again allowing flow of only background currentI_(B) by interposing resistor R, as shown in FIG. 1. A low current, lowenergy fuse explosion occurs. At the explosion, the metal transfer pulseis concluded, an arc is established and the voltage rises along line106. This closes switch SW. The main current is permitted to flow and itincreases along time constant portion 110 which starts from the lowlevel I_(B), illustrated to be less than 50 amperes in FIG. 4. The rapidincrease in welding current along line 110 toward the plasma level 112is a time constant curve. By using switch SW, current restoration doesnot swing upwardly above the normal plasma level I_(P). Such unwantedcurrent swing would cause control difficulties in restriking the arcsince the applied current would be substantially higher than needed foractual plasma ignition and maintenance. In accordance with this firstaspect of the present invention, the circuit SC is designated to dropthe current to a low level upon recognizing an imminent fuse explosion.Further, the welding current I_(W) is held to the low background levelI_(B) for a time T₃ which, in practice, is 1.0 ms. However, circuit SChas a control parameter so that whenever voltage V_(ARC) exceeds apreselected level, 10 volts in this instance, switch SW is closed. Sincethe arc voltage rises when neck N breaks, the rise in voltage alwaysoccurs prior to the time T₃. Consequently, the holding action of circuitSC for the time delay T₃ is only a fail-safe feature assuring thatcurrent I_(W) will ultimately be released for movement to plasma levelI_(P) at point or position 112 after being shifted to the backgroundlevel I_(B) along generally vertical line 114 just before the neckblows.

The prior art does not teach the concept of recognizing the necking,shifting to a low level background current value and then shifting backto the plasma level itself without drastically exceeding the plasmalevel I_(P) upon reestablishing the arc or plasma. Switching to a lowlevel value I_(B) is accomplished along the vertical line 114, which israpid enough to assure a reduction in current by a ratio of nearly 6:1.The prior art generally shifts the current along a controlled timeconstant curve and then releases the current at some selected timethereafter. Precise control of the welding current is accomplished bythe present invention by using Darlington switch SW which providesimmediate cutoff of I_(W) current, without a current storage in outputinductors or power leads which would result in the return curveexceeding substantially the plasma level 112.

Referring now to FIG. 5, one circuit for detecting top 52 of a metaltransfer pulse is illustrated as a di/dt detector, whereby as thecurrent shifts from a positive slope to a zero slope, an appropriateoutput from line 120 is processed by logic control L and is used to openswitch SW. This circuit takes the first derivative of the weldingcurrent I_(W) by differentiator 122. The derivative in line 124 isamplified by amplifier 126 and directed to input 128 of comparator 130.The output provides a logic signal indicating when K di/dt is at apreselected level. Other arrangements could be provided for detectingthe top 52 of pulse MT. The time between this top 52 and fuse breakpoint 54 is indicated by the spacing T_(N). As can be seen, by the scaleof the graph in FIG. 3, the time T_(N) is quite small. Consequently, assoon as a detect signal is generated in line 120, switch SW isimmediately opened. This plunges current I_(W) down to the backgroundcurrent level I_(B) along line 114. Current I_(B) is substantially lowerthan the plasma current I_(P), as indicated in FIG. 4.

INCIPIENT SHORT SPATTER REDUCTION

In accordance with another aspect of the present invention, spattercontrol SC is provided with a feature that reduces spatter caused by thephenomenon of incipient shorts. In accordance with this feature, asshwon graphically in FIG. 4 the arc or plasma current I_(P) isimmediately dropped to less than 50 amperes (i.e. the background levelI_(B)) when any type of short occurs. The voltage drops rapidly alongline 116, current I_(W) drops along line 118 by logic control L openingswitch SW. The welding current I_(W) is shifted to the backgroundcurrent I_(B) when the short first occurs. Background current I_(B) iscontrolled primarily by the value of resistor R and is held for apreselected time or cycle T₁ at I_(B). After this cycle or time T₁expires, switch SW is closed and the voltage at low level 120 commencesto rise to steady state level 102, as previously discussed. This drop ofcurrent as soon as a short is detected and holding the low level for aselected time eliminates incipient spatter.

To appreciate how the spatter control reduces spatter from incipientshorts certain technical features of the "incipient short" phenomenonare diagrammed in FIGS. 6A, 6B, 6C, 6D, 7 and 8. Referring now to FIG.6A, when ball B is formed on the end of wire 22, the ball, in someinstances, does not immediately attach to the molten weld pool bysurface tension. The ball may just touch the weld pool as illustrated inFIG. 2B. This is explained by recognizing that the weld pool is moltenmetal subjected to high arc currents as well as gravity andmagnetomotive forces. It resembles a wavy body of water. In manyinstances, the weld pool engages the ball B, as shown in FIG. 6B, for ashort time by a molten metal front striking ball B. The electrical andmechanical forces caused by wave fronts in the weld pool engaging ball Bsometimes drives the ball away from the undulating weld pool. This isshown in FIG. 6C. Consequently, there is a short when ball B touches theweld pool WP. This short can be immediately opened by mechanical forces,as shown in FIG. 6C. As this process continues and ball B is notcaptured by the weld pool, the ball continues to grow by the continuingmelting action at the end of wire 22. Ultimately, ball B can be drivenaway from the weld pool as a substantial mass of molten metal. This is aform of spatter. Due to the mass of the metal in the ball, themechanical spatter caused by these incipient shorts results in lumps ofmetal randomly deposited immediately adjacent the weld area. Thesespatter particles present substantial problems in cleaning and causewaste of weld material and welding energy. FIG. 7 illustrates thecurrent and voltage fluctuation during an incipient short when ball Btouches weld pool WP as shown in FIG. 6B. As the ball touches but doesnot transfer, it swings away as illustrated in FIG. 6C. The currentthrough ball B increases as the ball touches the weld pool. Thisincreased current causes forces as the arc is reestablished, in FIG. 6C,tending to drive ball B away from the weld pool. An incipient shortcauses the arc voltage to shift downwardly as the current increasesbetween the weld pool and the touching ball. This is shown in the lowercurve of FIG. 7. As soon as there is a break caused by mechanical forcesbetween the ball and the weld pool, the voltage moves verticallyupwardly along line 130 to the plasma arc voltage level. Since there isno large area contact for surface tension to draw molten ball B into theweld pool in an incipient short condition, the incipient short causesball B to grow and causes high mechanical forces to exert momentum tothe ball B tending to throw the ball away from the weld pool. Theincipient short phenomemon is schematically illustrated in FIG. 8wherein the weld current I_(W) experiences a series of incipient shortsbefore an active transfer short. In many instances, the molten ballremains molten after one or more incipient shorts and then goes into astandard transfer without causing large particle spatter. However, oftenthis does not occur. The incipient shorts cause ball B to grow and bepropelled outwardly by the high mechanical forces during the shortcircuit condition. If the effect of incipient shorts, illustrated assmall pulses 140, 142, in FIG. 8, is ignored, large particle spatterwill occur. This type of weld spatter presents more cleaning problemsthan spatter caused by breakage of fuse F in FIG. 2C. By using thepresent invention to drop the welding current to the background levelI_(B) as soon as a short is detected by a decreased voltage level,incipient shorts as shown schematically in FIGS. 6A, 6B, 6C, 6D, 7 and 8do not occur. The low level current I_(B) permits the metal contact areato increase and thus develop into a transfer short. There is no highcurrent to force an incipient short.

The "incipient short" graph portion of FIG. 4 illustrates what happensshould an incipient short occur as the result of some transientmechanical agitation, during time T₁ when the low level current I_(B) isapplied. Referring again to FIG. 4, the cycle or time T₁ is 1.0 ms,which time is substantially greater than incipient short durationdetermined by the width of pulses 140, 142 of the prior art as shown inFIG. 8. Assuming an incipient short occurs in use of the presentinvention, as soon as the short occurs, as shown in FIG. 6B, theoverriding control recognizes the reduction of the arc voltage to avalue below the preselected. trip value. Thus, the weld current isshifted downwardly by opening switch SW. In accordance with the presentinvention, the current flowing during any short is reduced thus reducingmechanical forces known to cause an incipient short condition. Thecurrent I_(B) is drastically below the main current level, thusfacilitating large area contact and transfer of the ball to the weldpool.

At point 150, shown in FIG. 4, should mechanical agitation move ball Baway from the weld pool, as shown in FIG. 6C, the arc will bereestablished at current level I_(B). This minimizes the energy evolvedwhen this small fuse explodes to reduce associated spatter at this smallfuse action. The arc voltage rapidly shifts upward along curve 152. Assoon as the arc voltage exceeded the preselected value, which is anoverride, trip condition valid at all times in control SC, switch SWwould be closed. The weld current I_(W) would then rise from thebackground current level I_(B) to the plasma current level I_(P).

Power supply 10 is a constant voltage machine set at a plasma voltage ofapproximately 20-30 voltage and it feeds a short in the general range of1-6 volts. When using this type machine, applying a main current byclosing switch SW allows the welding current to reach the normalunimpeded current level schematically illustrated as the increasing line100 in FIG. 4. When switch SW is opened, resistor R controls the weldingcurrent at the low, background level I_(B). As a recapitulation in thepresent invention, the main current is the normal unimpeded current andthe background current is the current controlled by the resistor.

GENERAL DESCRIPTION

Referring now to FIGS. 9 and 10, an advantage of employing the presentinvention is illustrated. In accordance with the invention, during themetal transfer, the current when switch SW is closed, rises, as shown bypulse MT (NEW), from a low level background current I_(B) to a detectedtop portion 52a, at which time switch SW is opened and the currentplunges along portion 180. This action is affected by the time constantincluding inductive reactance X_(L) of choke 190 which is usually insidepower supply 10. This inductance is also created by the length ofconductors 12, 14 extending from the power supply to nozzle 20 andworkpiece 30. The time to plunge from top 52a to the background currentI_(B), represented in the disclosed formula as I_(O), is relativelyshort. Thus, the current at the time of the fuse break will depend uponthe lead time of switch turn off before the fuse break. Lead times havebeen selected to cause the fuse break at approximately 10% of the heightat the top 52a when it is 200 amperes. This current reduction afterreaching top 52a, combined with the initial drop from plasma currentI_(P) to background current I_(B), results in a substantial reduction inthe separation energy when fuse F explodes or blows in response tofurther current flow through welding wire 22. Reduction of energy isthus accomplished by two phenomena. One is starting the transfer pulseat a low background level, i.e. in practice less than 50 amperes. Thesecond is using a Darlington transistor switch to immediately open theswitch and drive current to a level controlled by resistor R.

A power supply or welding system employing the present invention hasthree distinct stages determining the welding current. At first, thecurrent is the plasma level during the arcing condition. Secondly, thecurrent is depressed to a background level in response to a shortcircuit. Thirdly, at the end of a short circuit formed by metaltransfer, the current is again depressed. During the transfer of metal,the current is controlled by the electrical parameters of the circuit. Aschematic illustration of these various welding current stages during astandard metal transfer pulse is found in FIG. 11. This view correspondsgenerally to the pulse MT (NEW) in the lower portion of FIG. 4. Inspatter reduction control circuit SC, whenever the arc voltage exceedsthe preselected value, in this case, 10 volts, switch SW is closed todirect main current through the switch to the welding operation. Themain switch is closed during the pulse MT (NEW); however, backgroundvoltage V_(B) in portion 120 of FIG. 4 shifts upwardly to the generallyhorizontal portion 102.

DETECT IHHIBITOR AND TIME DELAY

FIG. 11 illustrates a time delay T₂ which is, in practice, 100microseconds and occurs immediately after cycle T₁. When this delaycycle is employed in the preferred embodiment of the present invention,it inhibits the circuit used to detect when the fuse is about to blowwhich is at a position near top 52a of curve 100. Such a circuit isillustrated in FIG. 5 to detect the change in sign of di/dt. FIG. 13discloses a circuit which measures and detects a preselected slope orderivative of the arc voltage, i.e. dv/dt. These detector circuits areinhibited until time delay T₂ expires. The reason for this feature isappreciated when considering portion 122 of the arc voltage immediatelyupon closing switch SW as shown in FIG. 4. The current causes a voltagerise that, in turn, produces a value for dv/dt which would erroneouslytrigger a dv/dt circuit because of the increase in current. This wouldopen switch SW erroneously. For that reason, a slight delay T₂ isprovided after expiration of cycle T₁ and closing of switch SW.

At the time of the transfer fuse and the drop of current along line 114before the fuse explodes, time T₃ is set. Time delay T₃ follows thedetection signal 200 caused by a detection of the point on curve 100just before fuse explosion or blow and has a maximum value. Time delayT₃ is shown in FIG. 11; however, it is seldom, if ever used. When switchSW is opened in response to a detect signal 200, the fuse blows with lowcurrent flow. An arc condition then forces the voltage along line 106 ofFIG. 4 to above the 10 volt limit, an action that closes switch SW andcauses the current to rise along line 110 to portion 112, which is theplasma current level. Immediate increase in voltage by interrupting theshort reconditions the system of the present invention to seek the nextmetal transfer cycle and to repeat control at the forward and rear endsof the welding current pulse. In the intermediate portion a floatingcondition exists along line 100, 102, as explained in connection withFIG. 4. Should the arc not cause a voltage override to close the switch,the switch will be closed after time T₃.

CURRENT CONTROL

Control of the background current occurs when a control signal 210 opensswitch SW as illustrated schematically in FIG. 12. That event placesresistor R (1.0 Ohms) directly into the welding circuit and removescurrent flow through switch SW. For the purpose of explaining certainaspects of the invention, it is assumed that the plasma current I_(P),as shown in FIG. 4, is approximately 300 amperes. When the switch isopened in response to the plunge in the arc voltage by a shortcondition, the 300 ampere current flowing when the switch is closed,develops 300 volts across resistor R. This high voltage is applieddirectly across Darlington transistor switch SW. To reduce the voltageacross switch SW during turn off, capacitor 192, having a value of 30mf, is connected in parallel with the resistor R. Consequently, resistorR immediately charges capacitor 192 toward the 300 volts of theresistor. In this example, it would require about 30 microseconds forcapacitor 192 to charge to 300 volts. The actual fall time of theDarlington collector current is less than 5 microsenconds; therefore,charging of capacitor 192 protects Darlington transistor againstexcessive power switching dissapation. A snubber or diode 194 preventsdischarge of capacitor 192 through the switch SW, should the switchclose when capacitor 192 is fully charged. Resistor R has at least twodistinct functions in accordance with the present invention. Theresistor establishes the magnitude of the background current I_(B). Ifthe voltage from power supply 10 is 20 volts D. C. as shown in FIG. 4,background current is 20 amperes calculated by dividing 20 volts by 1.0Ohm. As a second function, resistor R protects the Darlington transistorswitch SW against over voltage. A basic advantage of resistor R is thatwith resistor R in parallel with the Darlington switch Sw there is noneed for a second source of current to produce the background current.Further, there is no need for complicated circuitry attempting tocontrol current based on sensed conditions or parameters. Merely placingresistor R in parallel with Darlington switch SW produces the maincurrent, when the switch is closed, and the background current I_(B)when the switch is opened. This is the unique concept for providingcurrent levels which are employed in a unique system convenientlycontrolling spatter both of the electrical type caused by blowing offuse F and the mechanical type caused by the incipient short phenomenon.With capacitor 192 connected in parallel with resistor R, stored energyfrom the inductor 190 is dissipated when the switch is first opened andnon-conductive. This dissipation of energy occurs rapidly and isaccomplished before the switch SW is conductive or turned on asindicated by the dashed line 126 in FIG. 4. Since the energy has beendissipated, curve 110 can be gradually merged into the plasma currentlevel along a curve determined by the time constant of inductance orchoke 190. Without the resistor and its capacitor for dissipating storedenergy in the inductance, there would be no background current and thusno ionization formed by low current fuse action and, thus, cause andunstable arc start up or restrike.

PREFERRED CIRCUIT

FIGS. 13A and 13B, taken together, describe the preferred circuit forpracticing the invention, as best shown in FIGS. 4 and 11. Each of thecomponents is labeled; therefore, the circuitry is somewhatself-explanatory and only a brief description of the operation of thecircuit is sufficient to understand how the circuit complies with theparameters of the present invention. The arc voltage passes through anoise reduction circuit 300 having an output 302 labeled ARC VOLTAGE. Areference voltage in line 304 is combined with output 312 by comparator310 to produce a logic level in output 312. The logic in this linecontrols a monostable multivibrator or ball pulse generator 320 which istriggered when the logic in line 312 shifts to zero indicating that theARC VOLTAGE has decreased below the reference set in line 304. When thisoccurs, a 1.0 ms negative pulse appears in the Q output 322 forcontrolling NAND gate 330. A logic 0 in line 322 forces the output 322of NAND gate 330 to be at a logic 1. This opens switch SW by anappropriate circuit 340 the control of line 322 corresponds generallywith control signal line 210 of FIG. 12. In this manner, the switch SWis turned off for 1.0 ms corresponding to cycle T₁. Should the voltageincrease above the preselected value determined by reference line 304,reset generator 350 having an input 352 connected with line 312 shiftsoutput line 354 to a logic 0 , thus, clearing ball pulse generator 320and terminating the T₁ time pulse. A logic 1 is applied to the inputline 322 of gate 330. At the same time, the reset logic in line 354resets fuse time generator 360, if it is not already reset or cleared. Alogic 1 is thus applied, as a pulse, to line 362. This logic combineswith logic 1 on input 322 to produce a logic 0 in line 332. This turnsthe Darlington transistor switch SW on whenever arc voltage exceeds thepreselected value set, in the preferred embodiment, at 10 volts. Thisvoltage is selected to be greater than the voltage drop across wire 22to assure enough available voltage drop for proper detection.

As previously discussed with respect to FIG. 5, fuse F can be predictedwhen necking occurs as detected by dv/dt. In FIG. 13, including bothFIGS. 13A and 13B, a derivative of the arc voltage is compared to aconstant K for the purpose of predicting the fuse immediately uponestablishment of neck N. This is illustrated schematically in the upperportion of FIG. 4 at portion 104. To accomplish this comparison of dv/dtwith K, the preferred circuit in FIG. 13 uses a sample and hold concept.A sample oscillator 380 produces a series of sample pulses in output382. This is applied to the sample and hold circuit 390 to sample thevoltage at precisely spaced instances as the voltage is received on line302. The output of sample and hold circuit 390 is line 392 whichcompares a held sample voltage with an instantaneous voltage in line302. These two time spaced voltages (i.e. V_(N) and V_(N) -1) arecompared by comparator 400 to produce a differential in voltage ascompared to time. This differential signal is amplified by amplifier 410to create a differential of voltage with respect to time in line 412.The constant or K is selected by pot 414 which directs a selectedconstant through line 416 at the input of flip-flow 420 through line418. When the dv/dt exceeds slope K, flip-flop 420 is clocked. Thisapplies a logic 1 in line 422 producing a logic 0 in line 362 turningoff circuit 340 by a logic 1 in line 332. As so far described, when thevoltage is less than 10 volts, switch SW is open for a time T₁ which maybe overridden by a voltage increasing above 10 volts, such as wouldoccur in an incipient short. This will cause interruption of cycle T₁,as indicated by line 152 in FIG. 4. With the switch SW closed during ashort circuit, flip-flop 420 is awaiting a signal in line 418 indicatingthat dv/dt has exceeded constant K. When that occurs, a 1.0 ms pulseoccurs in line 362 holding line 332 at a logic 1. This is the time orcycle T₃, shown in FIG. 4, which cycle is generally never reached and isonly a feature providing back-up assurance that the system will shift tothe plasma current after a short. Before that happens, arc voltageincreases along line 106 to a level above a preselected value of 10volts. This causes reset generator 350 to reset ball pulse generator 320placing a logic 1 in line 322. At the same time, reset pulse in line 354produces a logic 1 in line 362. These two logic 1 inputs to gate 330produces a logic 0 in line 332, which turns on switch SW.

The inhibit feature indicated by time T₂ as shown in FIG. 4 assures thatthe derivative flip-flop 420 does not operate until 100 microsecondsafter T₁ is concluded. This is accomplished by another monostablemultivibrator 430 having an output 432 which is combined logically withthe logic in ball pulse line 322 by NAND gate 440. The output 442 islabeled HOLD CLEAR. It remains at a logic 1 until cycle T₁ expires asindicated by the logic in line 322. Thereafter the 100 microsecondnegative pulse on line 432 expires. This pulse is shown at the bottom ofFIG. 13B. As long as a logic 1 is maintained in line 442, flip-flop 420can not toggle to place a logic 1 in line 422. Consequently, time T₂ atthe end of cycle T₁ maintains the differential circuit inactive for ashort time allowing the voltage and current to stabilize and operate ina floating condition awaiting an ultimate detection at or near the top52a of metal transfer pulse MT (NEW).

FIGURE 14

Referring now to FIG. 14, the top graph depicts stages of a standardshort circuiting welding operation with arc jet forces and plasmacreated at the first stage I. The ball starts to grow in stages II andIII. Then an incipient short occurs with spatter at stage IV. After theincipient short has been terminated and a transfer is started, the ballstarts to neck at stage V. Then the fuse blows at stage VI. Current andvoltage curves for these stages are illustrated for a system without aspatter reduction device of the present invention and labeled "StandardProcess". The corresponding process curves using the present inventionare set forth at the lower portion of FIG. 14. As can be seen thepresent invention maintains a controlled plasma current as well ascontrolling the transfer pulse to nearly eliminate weld spatter.

In accordance with the present invention, there is a voltage responsiveoverride whereby the main current is applied by closing switch SWwhenever the voltage exceeded a preselected value, in the illustratedembodiment 10 volts. This override voltage is generally selected asabout half of the arc or plasma voltage.

Having thus defined the invention, the following is claimed:
 1. A devicefor reducing spatter when a welding power supply is employed fordepositing metal from a welding wire onto a workpiece by the shortcircuiting transfer mode wherein a welding current causes the weldingwire to alternate between a short circuit condition and an arccondition, with metal transfer during a short circuit conditionrequiring a transfer time T_(P) during which transfer time the weldingcurrent rises and then falls as a melted portion of said wire istransferred to said workpiece, said device comprising:(a) means forsensing the short circuit condition; (b) switching means having a firstswitched conductive condition wherein said welding current is allowed toreach the normal unimpeded current level controlled by said power supplyand a second switched non-conductive condition wherein said weldingcurrent is forced to flow through a low resistance resistor to limit thewelding current to a low level background current, said switching meanshaving low stored energy when shifted into either of said conditions;(c) shift means responsive to said sensing means for shifting saidswitch means into said second switched condition before said shortcircuit condition; (d) means responsive to said shift means for holdingsaid switch means in said second, low current condition for a cycle T₁beginning with said short circuit condition, wherein cycle T₁ has aduration with a maximum time less than said transfer time T_(p) ; and,(e) means operable during said short circuit condition for shifting saidswitch means to said first condition for the remainder of said shortcircuit condition whereby said welding current is allowed to reach thenormal impeded current level in an unrestricted manner unless said arccondition is established during said short circuit condition.
 2. Amethod for reducing spatter when a welding power supply is employed fordepositing metal from a welding wire onto a workpiece by a shortcircuiting transfer mode wherein a welding current causes the weldingwire to alternate between a short circuit condition and an arccondition, with metal transfer requiring a transfer time T_(P) duringwhich transfer time the welding current rises to a maximum value andthen falls, said method comprising the following steps:(a) detectingsaid short circuit condition; (b) shifting said welding current to abackground current value in response to said detecting step; (c) holdingsaid welding current generally at said background current value for apreselected time; (d) then allowing said welding current to reach thenormal unimpeded current level; (e) sensing the slope of the weldingcurrent or welding voltage during the transfer time T_(P) ; (f) shiftingsaid welding current to said background current generally when saidwelding current reaches a level at or just beyond said maximum value asdetermined by said sensing step; (g) again holding said welding currentgenerally at said background current value for a preselected time; and(h) again causing said second holding step to be terminated by allowingsaid welding current to reach the normal unimpeded current level.
 3. Adevice for reducing spatter when a welding power supply is employed fordepositing metal from a welding wire onto a workpiece by the shortcircuiting transfer mode wherein a welding current causes the weldingwire to alternate between a short circuit condition and an arc conditionwherein the welding current is at a plasma sustaining level, and withmetal transfer requiring a transfer time during which the weldingcurrent rises and then falls, said device comprising:(a) first shiftingmeans for shifting said welding current from said plasma level to a lowlevel background value when a short condition is to occur; (b) means forholding said welding current at said background value for a preselectedtime; (c) means for then allowing said welding current to rise from saidbackground value during said short circuit condition for transferringmetal to said workpiece; (d) means for sensing when said transferringmetal starts necking down preparatory to a fuse break; (e) means foragain shifting said welding current to said background value; and, (f)second shifting means for shifting said welding current back to saidplasma level awaiting the next successive short condition.
 4. A methodof reducing spatter when a welding power supply is employed fordepositing metal from a welding wire onto a workpiece by the shortcircuiting transfer mode wherein a welding current causes the weldingwire to alternate between a short circuit condition and an arc conditionwith the welding current being at a plasma sustaining level, and withmetal transfer requiring a transfer time during which the weldingcurrent rises and then falls, said method comprising the followingstep:(a) shifting said welding current from said plasma level to a lowlevel background value as a short condition begins; (b) holding saidwelding current at said background value for a preselected time; (c)then allowing said welding current to rise from said background valueduring said short circuit condition for transferring metal to saidworkpiece; (d) sensing when said transferring metal starts necking downpreparatory to a fuse break; (e) shifting said welding current to saidlow level background value in response to said sensing step; and, (f)shifting said welding current back to said plasma level awaiting thenext successive short condition.